package tut.fileLock;

import java.nio.ByteBuffer;
import java.nio.IntBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;
import java.io.RandomAccessFile;
import java.util.Random;

/**
 * Test locking with FileChannel. Run one copy of this code with arguments
 * "-w /tmp/locktest.dat" and one or more copies with "-r /tmp/locktest.dat" to
 * see the interactions of exclusive and shared locks. Note how too many readers
 * can starve out the writer. Note: The filename you provide will be
 * overwritten. Substitute an appropriate temp filename for your favorite
 * operating system.
 *
 * Created April, 2002
 *
 * @author Ron Hitchens (ron@ronsoft.com)
 * @version $Id: LockTest.java,v 1.2 2002/05/19 04:55:45 ron Exp $
 */
public class FileLockTest2 {
	private static final int SIZEOF_INT = 4;
	private static final int INDEX_START = 0;
	private static final int INDEX_COUNT = 10;
	private static final int INDEX_SIZE = INDEX_COUNT * SIZEOF_INT;

	private ByteBuffer buffer = ByteBuffer.allocate(INDEX_SIZE);
	private IntBuffer indexBuffer = buffer.asIntBuffer();
	private Random rand = new Random();

	public static void main(String[] argv) throws Exception {
		boolean writer = false;
		String filename;

		if (argv.length != 2) {
			System.out.println("Usage: [ -r | -w ] filename");
			return;
		}

		writer = argv[0].equals("-w");
		filename = argv[1];

		RandomAccessFile raf = new RandomAccessFile(filename, (writer) ? "rw"
				: "r");
		FileChannel fc = raf.getChannel();

		FileLockTest2 lockTest = new FileLockTest2();

		if (writer) {
			lockTest.doUpdates(fc);
		} else {
			lockTest.doQueries(fc);
		}
	}

	// ----------------------------------------------------------------

	// Simulate a series of read-only queries while
	// holding a shared lock on the index area
	void doQueries(FileChannel fc) throws Exception {
		while (true) {
			println("trying for shared lock...");
			FileLock lock = fc.lock(INDEX_START, INDEX_SIZE, true);
			int reps = rand.nextInt(60) + 20;

			for (int i = 0; i < reps; i++) {
				int n = rand.nextInt(INDEX_COUNT);
				int position = INDEX_START + (n * SIZEOF_INT);

				buffer.clear();
				fc.read(buffer, position);

				int value = indexBuffer.get(n);

				println("Index entry " + n + "=" + value);

				// Pretend to be doing some work
				Thread.sleep(100);
			}

			lock.release();

			println("<sleeping>");
			Thread.sleep(rand.nextInt(3000) + 500);
		}
	}

	// Simulate a series of updates to the index area
	// while holding an exclusive lock
	void doUpdates(FileChannel fc) throws Exception {
		while (true) {
			println("trying for exclusive lock...");

			FileLock lock = fc.lock(INDEX_START, INDEX_SIZE, false);

			updateIndex(fc);

			lock.release();

			println("<sleeping>");
			Thread.sleep(rand.nextInt(2000) + 500);
		}
	}

	// write new values to the index slots
	private int idxval = 1;

	private void updateIndex(FileChannel fc) throws Exception {
		// "indexBuffer" is an int view of "buffer"
		indexBuffer.clear();

		for (int i = 0; i < INDEX_COUNT; i++) {
			idxval++;
			println("Updating index " + i + "=" + idxval);

			indexBuffer.put(idxval);

			// Pretend to this is really hard work
			Thread.sleep(500);
		}

		// leaves position and limit correct for whole buffer
		buffer.clear();
		fc.write(buffer, INDEX_START);
	}

	// ----------------------------------------------------------------

	private int lastLineLen = 0;

	// Specialized println which repaints the current line
	private void println(String msg) {
		System.out.print("\r ");
		System.out.print(msg);

		for (int i = msg.length(); i < lastLineLen; i++) {
			System.out.print(" ");
		}

		System.out.print("\r");
		System.out.flush();
		lastLineLen = msg.length();
	}
}